Fluid flow equipment including pipes, valves, and pumps are common in the utility, refinery, manufacturing, chemical and petrochemical industries. The mechanical workings of such valves and pumps are housed in casings through which rotary or reciprocating shafts extend. For example, the shaft of a rotary pump operatively connects a motor on the exterior of the casing to an impellor on the interior of the casing. The shafts rotate or reciprocate in response to a number of specific stimuli, including a knob turned by hand, a motor, or an impeller driven by fluid flowing in the equipment.
Thus, there are at least three openings in a pump or valve casing: (1) an opening for an inlet pipe by which fluid is delivered; (2) an opening for an outlet pipe by which fluid is discharged; and (3) an opening for the shaft. Various types of seals prevent leakage of fluid from the pump or valve casing. The two fluid openings for the inlet and the outlet pipes are sealed conventionally. The shaft projects through the casing in an area known as the "stuffing box" or the "packing box." The terms "stuffing box" and "packing box" are interchangeable, and derive from the method of preventing fluid leakage by stuffing or packing a material around the shaft to provide the seal. The packing material is often composed of woven or braided fibers formed into coils, spirals or rings. The packing material is stuffed around the shaft so that no fluid can escape the casing along the shaft. A lubricant is often impregnated in the packing material to facilitate installation and to reduce friction on the packing material.
Rotary and reciprocating shaft-equipped pumps, valves, compressors, agitators and the like, interact with a variety of fluids. Such fluids may be as harmless as cool water or as dangerous as a radioactive, superheated acid. Preventing leakage of any fluid from the opening for the shaft is important. The cost of any such leakage can range from the loss of fluid and operating time for repair of the leak, to significant environmental damage and loss of life.
For example, consider a pump in a nuclear fueled steam generating plant. In nuclear reactors, pumps are used to circulate a coolant (oftentimes water) across nuclear fuel elements. The coolant and nuclear fuel are placed together in a pressure vessel. Piping from the pressure vessel delivers the coolant, heated by contact with the nuclear fuel, to a heat exchanger. The heat exchanger extracts the heat from the coolant. The piping thus forms a continuous loop between the pressure vessel and the heat exchanger so that the coolant is continuously recycled. As a result, radioactivity is safely contained within this closed system. Pumps are often provided between the pressure vessel and the heat exchanger to deliver the coolant. Any leakage from the pump destroys the closed system and permits radioactive coolant to escape. Failure of a seal in this example will not only result in the discharge of a toxic material into the environment, but could cause an explosion or fire.
In addition to actual damage caused by leaks, profits and the health and welfare of employees are affected. Many industrial processes require large amounts of time to regain normal operation. Frequent shutdowns of the process greatly affect production capability. Thus, having to shut down a plant for any period of time in order to replace worn or damaged packing in the stuffing box reduces operating time and, correspondingly, reduces profit. Moreover, workers are often at risk in replacing such worn packing. For example, the packing in a pump and valve becomes saturated with the fluid being sealed. The packing in a pump used to circulate coolant in a nuclear reactor will be exposed to radioactivity from the coolant. A worker who removes old and worn packing from such a pump is, for a time, exposed to the radiation contained in the fluid and saturated in the packing. Accordingly, frequent replacement of the packing material in the stuffing box is not desirable. Moreover, it is preferred that all steps be taken to minimize the risk of such radiation exposure.
Rotating and reciprocating shafts are difficult to seal. In operation, such shafts are capable of both radial and axial displacement. Radial displacement typically results from manufacturing inaccuracies. Axial displacement results from different thermal expansions produced through normal operation of the shaft. Furthermore, the stuffing box environment is less than ideal. Conditions are constantly changing. The packing may be required to withstand high temperatures and pressures one minute and low temperatures and pressures the next. Shaft speeds may also vary. The surfaces of the shaft in the stuffing box are often pitted and rough. Very slight defects in the arrangement or condition of a stuffing box can prevent proper pump operation.
Various types of packing for a stuffing box are known in the prior art. Each of these packings attempts to be responsive to the foregoing considerations. The packing must be somewhat plastic so that it can extrude enough to seal rough or uneven surfaces. The packing must be resilient in order to adapt to changing conditions without failing or damaging the shaft. However, in trying to provide flexibility, some packings sacrifice resiliency. Others, in trying to resist extrusion, sacrifice flexibility sufficient to conform to uneven or rough surfaces within the stuffing box. Still other packings are flexible, resilient and minimize friction, but do not provide a long-lasting seal so as to avoid frequent replacement.
Soft packing is a common shaft seal, and is generally made from asbestos, fabric, hemp or rubber fibers woven into strands and formed into a braided spiral. Soft packing is inexpensive and offers several desirable features. The softness of the packing allows it to absorb energy without damaging the rotating shaft. Soft packing is also very flexible and readily conforms to the area to be sealed.
Soft packing, however, has several disadvantages. One problem is short life. Soft packing is easily worn by friction and easily damaged, therefore requiring frequent replacement. Soft packing may be impregnated with graphite or lubricating oils to reduce friction between the shaft and the packing, but such lubricants quickly dissipate and are not very effective in overcoming the short life problem. Thus, soft packings are best suited for low shaft speed applications involving non-caustic and non-abrasive fluids. Yet another problem with soft packing is a lack of resiliency. After being compressed and extruded, soft packings are unable to re-expand to effectuate a seal. Resiliency, conventionally defined as the ability of packing to re-expand, is important to enable the packing material to adjust to changing conditions. Lack of such resiliency, as in the case of a soft packing, results in frequent adjustment or replacement for the packing.
U.S. Pat. No. 3,404,061 teaches a sealing material made from expanded graphite. One common use of such material is to wind a length of flexible tape made therefrom onto a mandrel to form a solid annulus of appropriate size to pack the stuffing box. Thus, the expanded graphite tape is formed as a seal. Packing made from expanded graphite is flexible and conforms to uneven surfaces. The graphite material makes the packing self-lubricating, thereby minimizing friction between the shaft and the packing. With such self-lubricating packings, the lubricant does not dissipate with time. Expanded graphite packing also absorbs energy without excessive damage to either the packing or the shaft.
The principal problem with expanded graphite packings is a lack of resiliency and excessive extrusion under high temperatures and pressures. Solid graphite packings are not able to withstand high pressures since they lack the internal strength to resist extrusion and are unable to re-expand after compression. In addition, expanded graphite packings require frequent adjustment under normal conditions due to the low resiliency of the graphite. The graphite packings are easily compressed, thereby contributing to the low resiliency problem. As a result, normal rotation or reciprocation of the shaft can compress the graphite and create leaks.
A further problem with soft packings (and expanded graphite packing in particular) is that they are difficult to extract from the stuffing box when replacement is necessary. Soft packing can extrude to such an extent that it melds to the walls of the stuffing box, making removal difficult. Those skilled in the art will appreciate that the typical stuffing box provides an annular recess about the shaft, into which the packing is stuffed. The recess is capped by a gland follower. The gland follower is secured to the casing, known as the gland of the stuffing box, by one or more bolts. Thus, the more torque applied to the gland bolts, the greater the downward pressure applied to the packing by the gland follower. Tightening the gland bolts compresses the packing in the stuffing box to effect the seal.
Generally speaking, there are three conditions that result in leakage: packing consolidation; bolt creep; and improper loading.
Packing consolidation occurs naturally, and refers to the packing's tendency to settle, wear, and loosen over time. A number of factors contribute to this condition, including the constant rotation of the shaft, changes in temperature of fluids flowing through the equipment, and the age and material of the packing itself. Soft packing is particularly susceptible to consolidation.
Bolt creep is a condition wherein the gland bolts are moved upward due to the expansion and contraction of the gland follower and the casing. Such expansion and contraction often results from a change in operating temperatures and pressures. Valves and pumps in various industries often operate under conditions ranging from cyrogenic to superheated temperatures, and normal to extreme pressures and vacuums. Bolt creep reduces the pressure applied by the gland follower on the packing.
Improper loading is a condition wherein the compression exerted by the gland follower on the packing is insufficient to effect a seal. Packing consolidation and bolt creep are contributing elements of improper loading, because both reduce the compressive force applied by the gland follower on the packing. But inaccurate torquing of the gland bolts by workers also causes improper loading. Such inaccurate torquing may be the result of human errors. However, it is recognized that the torque wrenches used by workers are often inaccurate, resulting in improper loading. Leaks thus occur from the outset because the load on the packing is insufficient to achieve or maintain a seal.
Fluid leakage along the shaft of valves and pumps has long been recognized as a serious problem in power and industrial plants. In recognition of this problem, various attempts have been made to obtain leak-free performance and reduce maintenance requirements for a pump or a valve. For example, improved packing materials were developed for a larger range of temperatures, better chemical resistance, and improved coefficient of expansion characteristics. Torque values were established for the bolts connecting the gland follower to the stuffing box. (Installers follow such specifications to apply a proper load to the packing to achieve a seal, but as discussed above, may not attain a proper load because the torque wrenches are inaccurate.) Several companies have initiated routine maintenance programs that include re-torquing of gland follower bolts. Such retorquing is done frequently because of the significant risk posed by improperly loaded gland bolts and the resulting leakage of fluid from the apparatus. The costs of repairing damaged equipment and cleaning up spent fluids are also of concern. But generally, the majority of the equipment does not need such maintenance. Such maintenance programs include all equipment, however, in order to correct the torque on the relatively few pieces of equipment for which packing compression is lessened (as a result of bolt creep, packing consolidation or previous improper loading) to an extent that leaking has occurred or could occur.
Another attempt to obtain leak-free performance and reduce maintenance requirements involves liveloading of the gland follower. Liveloading refers to the mounting of compressed springs on the gland follower whereby a constant pressure is exerted on the gland follower to insure a constant compressive force is exerted on the packing. As the packing consolidates or the gland bolts loosen, the spring pressure moves the gland follower towards the stuffing box to maintain the integrity of the packing.
Belleville washers are one type of spring typically used to cushion heavy loads with short motion. Uncompressed belleville springs or washers typically take the form of a disk with an open center. In contrast, compressed belleville washers are flat. A significant amount of force is required to compress or flatten the uncompressed belleville washers. Belleville washers installed on the gland bolts of pump and valve stuffing boxes maintain the force exerted by the gland follower on the packing. As the packing consolidates or the gland bolts loosen, the belleville washers decompress and maintain the load on the packing. The gland follower essentially becomes self adjusting in response to the packing's condition to maintain a proper load on the packing and thereby maintain a seal.
Liveloading a gland follower is difficult in many situations. It is particularly difficult to retrofit valves for liveloading for a number of reasons. Replacement of bolt studs may be necessary because the studs are not long enough to accomodate a sufficient number of uncompressed belleville washers and the nut that conventionally maintains the gland follower. Those skilled in the art will appreciate that uncompressed belleville washers occupy more space than compressed washers. Accordingly, the gland bolts must often be extruded and replaced with longer bolts. This is particularly expensive in nuclear power plants, not only because expensive high grade steel material must be used to manufacture the extended bolts, but also because a significant amount of paperwork detailing the change must be prepared and filed with the various regulatory agencies and manufacturers involved with the equipment and nuclear power plants. Also, health and safety inspectors at nuclear plants track carefully the amount of radiation to which workers are exposed because there is a limit to the amount of radiation a worker may receive. Additional workers thus may be needed for simple, yet time-consuming projects.
Another reason that liveloading is difficult is because achieving the right load on the belleville washers is expensive and difficult. The retaining nut must be torqued on the bolt to a specific degree to achieve and maintain a seal. Proper torquing of the washers, even using torque wrenches, takes a long time. In a nuclear plant, any additional maintenance time increases the workers' exposure to radiation from the fluid. Torque wrenches are recognized as inherently inaccurate. Engineers at nuclear plants in particular are uncomfortable relying on such tools to achieve a proper torque.
Yet another reason that liveloading is difficult is because belleville washers are difficult to install about a gland bolt. Aside from being a time consuming operation, the component washers are small in size and difficult to manipulate. Workers in heavily radiated areas must wear several sets of gloves and a respirator. Gloves make such small objects difficult to handle and position over a bolt. The respirator makes it difficult to see. If a single belleville washer is dropped and lost, work may be delayed for hours.
A further reason that liveloading is difficult is that belleville washers, once placed on a gland bolt and even when properly torqued, may slip laterally and hang or catch on the bolt. This causes hysteresis, a retardation of the self-adjusting effect of the belleville washers on the gland follower.
Thus, there exists a need in the art for an apparatus for liveloading, and for a method of preparing an apparatus for liveloading, the gland follower that is free of the problems typically experienced when liveloading valves, pumps and the like in power and industrial plants.